The proton charge radius

The proton is responsible for about half the mass in our personal lives. So it is perhaps surprising that little is known about how it got here or how it is bound from its fundamental quark and gluon constituents. This uncertainty is the largest source of error to extracting some fundamental constants of nature. One observable that probes the internal structure of the proton and tests our theoretical description of it is the proton charge radius. The radius is sensitive to quantum electrodynamics and quantum chromodynamics and helps to constrain new physics. This review presents the current status among the four types of experiments to measure the radius: electron scattering, muon scattering, electronic hydrogen spectroscopy, and muonic hydrogen spectroscopy.